1. Field of the Invention
The invention relates to a controller for an unbalanced mass-adjusting unit in a vibration exciter.
2. Description of the Related Art
Vibration exciters of this type are known in particular for use in ground-compaction machines, such as vibration plates or rollers. Conventionally, two—but also several—unbalanced shafts are driven in a positive manner rotating in opposite directions, wherein the superposition of the centrifugal forces of each individual unbalanced shaft serves to produce a resulting force which depending upon the design of the vibration exciter can be adjusted with respect to its value and/or direction. Vibration exciters of this type have been known for a long time particularly in the case of vibration plates, so that a further description thereof would be superfluous.
In order to adjust the desired value or the direction of the resulting force vector, it is necessary to be able to change the relative positions of the unbalanced shafts with respect to each other or of the unbalanced masses in relation to the shafts which support them. To this end, either the counter-rotating unbalanced shafts can be adjusted with regard to their mutual phase position or the position of the unbalanced mass on the shaft supporting it can be changed. For this purpose, unbalanced mass-adjusting units are known, wherein a transverse spigot which can be adjusted in an axial manner by an adjusting piston in an unbalanced shaft which is formed as a hollow shaft passes through a longitudinal slit in the wall of the hollow shaft and engages into a spiral groove on the inner side of a hub which supports an unbalanced mass. In the event that the transverse spigot is displaced axially, the transverse spigot is guided in the longitudinal slit, whereby the effect of the spiral groove causes the hub to change its relative position on the shaft which supports it. Therefore, it is possible to achieve the desired, above-described adjustment of the phase position. This structure has also been known for a long period of time and is illustrated e.g. in EP-A-0 960 659.
In the case of vibration exciters, the adjusting units thus serve to adjust the phase positions of unbalanced masses with respect to each other in order to reverse the direction of travel or in order to change the so-called “m·r-value”. They are based upon an hydraulic adjustment by means of a piston which is influenced on one side and which in the event of falling oil pressure is reset either by way of a spring device or by way of the restoration moment exerted by means of the unbalanced masses.
EP-A-0 960 659 also describes a controller for an unbalanced mass-adjusting unit, wherein the adjusting piston is hydraulically coupled to a reference piston. The position of the reference piston is controlled by way of a pilot valve. In turn, the position of the pilot valve is governed by forces which act upon the valve body from two sides, namely on one side by means of a force which can be predetermined by the operator and can be introduced by way of a spring into the valve body, and on the other side by means of a force which is exerted on to the valve body by means of the reference piston by way of a spring. The known controller is difficult to adjust and does not render it possible to perform different control algorithms. In order to carry out an adjustment or effect a change in the adjustment, it is necessary on each occasion to change the characteristic curves of the sets of springs which influence the valve body, which constitutes either substantial structural or substantial assembly outlay.
In the case of more complex vibration exciters, in which in addition to adjusting the phase position it is also possible to adjust the mr-value and thus under certain circumstances several unbalanced mass-adjusting units are provided, it is necessary to obtain a reliable signal relating to the respective position of the adjusting piston and therefore of the unbalanced masses in the exciter which are adjusted by said adjusting piston.